1. Field of the Invention
The present invention relates to a liquid fuel cell system and, more particularly, to a liquid fuel cell system used as a power supply of an electronic apparatus having a load which fluctuates with time, and a boosting unit of the same.
2. Description of the Related Art
Recently, a liquid fuel cell system which generates electric power by directly extracting protons from liquid fuel such as methanol or ethanol has been developed. Fuel cells of this type require only small fuel volumes because no modifier is necessary, so they are expected as power supplies of portable electronic apparatuses such as portable personal computers, personal digital assistants (PDAs), and video/audio players.
The electromotive force of one stack of a liquid fuel cell system such as a direct methanol fuel cell (DMFC) is small. To use this liquid fuel cell system as a power supply of a portable electronic apparatus, therefore, it is necessary to stack a large number of stacks in series or to boost the output voltage. If a large number of stacks are stacked in series, the size of the system increases. Accordingly, the use of a boosting circuit such as a DC-DC converter is generally being studied.
Unfortunately, the internal resistance of a fuel cell itself is high in the liquid fuel cell system. Therefore, when the liquid fuel cell system is used as a power supply of a portable electronic apparatus having a load which fluctuates with time, the output voltage of a fuel cell significantly lowers owing to the operation of a boosting circuit if the load increases. In some cases, the fuel cell shuts down.
On the other hand, an automobile fuel cell system having a function of preventing excess power supply has been proposed in order to control load fluctuations. As disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-44807, this system uses a hybrid power supply method which combines a fuel cell and secondary battery. If the output of the fuel cell exceeds a predetermined allowable value, an output command signal value of a DC/DC converter is decreased in accordance with the load increase. This prevents excess power supply exceeding the rated output from the fuel cell to the load.
In the conventionally proposed system described above, it is possible to prevent excess power supply exceeding the rated output from the fuel cell to the load. However, in a region below the rated output, the output of the fuel cell still fluctuates in accordance with the fluctuation in load. Accordingly, the inconvenience that the fuel cell shuts down is not eliminated, and this decreases the stability and power generation efficiency of power supply.
It is an object of the present invention to provide a liquid fuel cell system always capable of stable power supply against load fluctuations, and also capable of holding a high power generation efficiency, and a boosting unit of the same.